MONDAY, Oct. 22, 2018 (HealthDay News) -- The U.S. Drug and Food Administration has repeatedly warned manufacturers that many dietary supplements contain dangerous, experimental stimulants. But according to a new report, 75 percent of supplements tested still contain the compounds.

"Consumers turn to supplements for safe, natural ways to increase energy, improve workouts or lose weight," said study author Dr. Pieter Cohen, an associate professor with Harvard Medical School. "[But] what most consumers don't know is that supplements can be sold as if they give you energy, help you lose weight or just about anything, as long as the supplement does not claim to cure or treat disease."

His team's findings centered on four unapproved stimulants: DMAA, DMBA, BMPEA and oxilofrine.

The four have become replacements for the stimulant ephedra, which the FDA banned from supplements in 2004 following reports that it raised the risk for heart attack, stroke and death.

Between 2013 and 2016, the FDA found that 12 different supplement brands contained one or more of the four unapproved stimulants. But despite public notice warnings from the agency, three-quarters of the supplements still contained at least one prohibited stimulant in 2017. And half contained two or more.

The finding raises fresh concerns about supplement safety, and comes on the heels of another troubling analysis published just last week by the California Department of Public Health's Food and Drug Branch.

That investigation revealed that the FDA had issued more than 700 warnings over the last decade about potentially hazardous ingredients found in supplements promoted as sexual, weight-loss and muscle growth aids.

But because the FDA classifies dietary supplements as a food -- rather than as drugs -- supplement manufacturers do not have to prove a supplement is safe or effective before selling it to the public.

If, however, the FDA ultimately determines that a supplement already on the market is potentially hazardous, it can recall the product or issue a "public notice" concerning problematic ingredients.

In a letter published online Oct. 22 in the journal JAMA Internal Medicine, Cohen and his colleagues point to an earlier study that suggests that FDA recalls, for one, are largely ineffective.

The latest investigation focused on the effectiveness of public notice warnings, and found equally poor results.

"The FDA seems to imagine that if they simply request that firms remove an experimental stimulant from commerce, the stimulant will be removed," Cohen said. "Clearly this is wishful thinking on the part of the FDA."

Cohen noted that he and his associates conducted two analyses of 12 supplements previously issued public notices for unapproved ingredients.

The first analysis took place in 2014. At that time, all 12 supplements contained at least one of the four prohibited stimulants.

The second analysis took place in 2017. At that point, nine of the 12 brands contained at least one prohibited supplement and six contained two.

The team also pointed out that although DMBA had not been found in any of the 12 supplements in 2014, it was found in a third of the supplements in 2017, two years after the FDA issued a public notice raising concern about the ingredient.

"Until the law is reformed and the FDA aggressively enforces the law, these potentially dangerous ingredients will likely remain in supplements," said Cohen.

Dr. Mitchell Katz is president and CEO of NYC Health and Hospitals in New York City, and author of an accompanying editorial. He suggested that the upshot is that the FDA is hamstrung in what it can do.

"The FDA is not allowed by federal law to investigate a product before it is marketed," Katz noted. "Therefore, all that the FDA can do under current law is respond to complaints and issue guidelines of what can and cannot be in supplements."

The bottom line, said Katz, is that "people should know that the supplements they are taking are not tested [and] may contain substances that are not on the label."